European hotels commnonly have higher rates for two guests, part of it to do with the cost of the included breakfast, I presume. I can remember staying in European rooms into which two folks of normal dimensions could not have fit (and what was once a "Maid's room" in an old NYC hotel decades ago).

Certainly, for meetings and other group events, major US hotel chains use two tier, Single or Double, prices a majority of the time.

On the other hand, in today's slow market, I'm surprised you were not offered the choice of a modest additional fee for youtr SO to join you. I don't blame the hotel, for you seem to have requesteda "Single".